fix(server): emit fixed workspace fence headers (#26443)

packages/opencode/src/server/routes/instance/httpapi/middleware/fence.ts

@@ -0,0 +1,20 @@
+import { Flag } from "@opencode-ai/core/flag/flag"
+import { Effect } from "effect"
+import { HttpRouter, HttpServerRequest, HttpServerResponse } from "effect/unstable/http"
+import * as Fence from "@/server/shared/fence"
+
+const ignoredMethods = new Set(["GET", "HEAD", "OPTIONS"])
+
+export const fenceLayer = HttpRouter.middleware<{ handles: unknown }>()((effect) =>
+  Effect.gen(function* () {
+    const request = yield* HttpServerRequest.HttpServerRequest
+    if (!Flag.OPENCODE_WORKSPACE_ID || ignoredMethods.has(request.method)) return yield* effect
+
+    const previous = Fence.load()
+    const response = yield* effect
+    const current = Fence.diff(previous, Fence.load())
+    if (Object.keys(current).length === 0) return response
+
+    return HttpServerResponse.setHeader(response, Fence.HEADER, JSON.stringify(current))
+  }),
+).layer